This invention relates in general to the manufacture of cast aluminum products and, in particular, to an improved method for producing a cast aluminum vehicle wheel.
In producing cast aluminum alloy products, such as vehicle wheels, it is generally necessary, after the initial casting operation, to subject the casting to a series of metal treatment steps in order to produce a casting having the desired tensile strength, yield strength, elongation, and fatigue strength properties. These steps include: (1) a "solution heat treatment" (SHT) process and (2) an "aging" (i.e., precipitation hardening) process. In the SHT process, an aluminum alloy casting is first heated to a "solution" temperature of about 1000.degree. F. for a predetermined time such that certain soluble constituents contained in the alloy (such as age hardening constituent magnesium silicide Mg.sub.2 Si) are dissolved into "solid solution". The casting is then immediately and rapidly cooled (such as by quenching in a water bath) to retain the constituents in solid solution. This prevents rapid precipitation of the associated constituents which would otherwise occur if the casting were allowed to slowly cool through a certain temperature range. Next, during the "aging" process, the hardening constituents are precipitated out of the solution in a controlled manner to produce a casting having the desired mechanical properties. The aging is effected either "naturally" at room temperature over a period of at least 10-12 hours, or it can be "accelerated" by heating the casting to an elevated temperature for a shorter period of time (e.g. 450.degree. F. for 30 minutes).
The conventional method of producing gravity-cast aluminum wheels includes initially pouring a suitable molten aluminum alloy, such as A356 aluminum, into a mold through its gate channel until the molten alloy flows upwardly through one or more mold risers. After the molten alloy has completely solidified, the casting is removed from the mold, at which time it can be degated (i.e., the portion of the casting which solidified in the gate channel is cut off) and quenched in water to cool the casting to room temperature. The casting is then derisered (i.e., the riser portions of the casting are removed) and subjected to fluoroscope inspection to locate any obvious casting defects.
Next, a group of wheels (typically between about 70 and 350), are loaded onto racks and subjected to a "batch" solution heat treatment process. The batch solution heat treatment process is effected by placing the racks in a large gas-fired or electrical-resistance forced air convection oven. In the convection oven, the castings are heated to a desired "solution" temperature (approximately 1000.degree. F.) and are maintained at this temperature for approximately 2 to 8 hours. Following heating, the batch of wheels are immediately quenched in water to rapidly cool the wheels. Following cooling, the wheels are machined and painted and/or clear coated, during which time they are naturally aged at room temperature.
One of the problems associated with the above method for producing cast aluminum wheels relates to the amount of "work-in-process" which occurs as a result of the long process times. It is known that once a casting is heated to the correct "solution" temperature, proper solution heat treatment will occur within about 5 minutes. However, since a large number of wheels are heated during the batch solution heat treatment process, it is difficult to maintain even and uniform temperatures in all the wheels. Thus, to ensure that all the wheels are properly heat treated, the time to solution heat treat the wheels is usually at least two hours.
On a conservative estimate, the above described series of metal treatment steps, beginning with the casting of the wheel and through both the solution heat treatment and aging processes, require at least 12 hours to produce the wheels and more realisticly, takes closer to about 24 hours. Thus, any defect in the wheels (which is typically located during machining) is not readily discoverable until a relatively large number of wheels are "in process". As a result, a large number of wheels can be produced before a casting defect is discovered. In addition, since the wheels are cooled to room temperature prior to being solution heat treated, additional energy (and time) is needed to reheat the wheels up to the specific temperature necessary for solution heat treatment.
One alternate method for producing aluminum alloy castings, such as a piston, is disclosed in U.K. Patent No. 390,244. According to the method of this patent, an aluminum alloy material is cast in a mold and removed from the mold while the temperature is above 662.degree. F. (350.degree. C.). The casting is immediately placed in an oven maintained at a specific elevated temperature in the range of 788.degree. to 968.degree. F. (420.degree. to 520.degree. C.), and remains in the oven for a time period between 10 and 30 minutes. Following heating, the casting is quenched in water, and then either naturally or artificially aged.
Other methods for producing a cast article of aluminum alloy material are disclosed in U.S. Pat. Nos. 4,420,345 and 4,419,143, both issued to Ito et al. According to the methods in these patents, an aluminum-silicon-magnesium alloy or an aluminum-silicon-copper-magnesium alloy containing 0.03 to 1.0% by weight of antimony is cast into a mold. Then, after the casting has completely solidified but before the temperature has fallen below 842.degree. F. (450.degree. C.), the casting is placed in a heating furnace maintained at a specific elevated temperature in the range of 896.degree. to 1022.degree. F. (480.degree. to 550.degree. C.), for a time period of less than 2 hours. Following heating, the casting is quenched in water and then subjected to an artificial aging process at a specific elevated temperature for less than 12 hours.
The methods disclosed in all of the above patents reduce the time to solution heat treat the cast aluminum article by not allowing the casting to cool below a certain temperature before initiating solution heat treatment. However, they all still utilize forced air convection furnaces to solution heat treat and/or artificially age the castings. Some drawbacks of forced convection furnaces include lengthly heat-up time before reaching processing temperature, difficulty in obtaining uniform temperature distribution, and sometimes inconsistent product quality.
Recently, electric infrared (IR) heating systems have received increasing attention in certain industrial applications. In an infrared heating system, a product is heated by generating electromagnetic radiation waves at a specific frequency and intensity, and directing these waves at the product. The particular frequency (i.e., wavelength) and intensity are selected in accordance with the particular heating requirements of the product. While infrared heating systems are used in a variety of different industrial applications, they are primarily used to dry and/or cure products with layers or thin films on their surfaces.